1. Field of the Invention
The present invention relates to a sheet conveying apparatus provided in an image forming apparatus such as a printer, a facsimile machine, a copying machine and a multifunction machine having combined functions thereof, and to an image forming apparatus which includes the sheet conveying apparatus.
2. Description of the Related Art
Examples of systems employed in image forming apparatuses include the electrophotographic system, the offset printing system and the inkjet system. Color image forming apparatuses employing the electrophotographic system will be described here as an example to describe the related art. Color image forming apparatuses can be mainly classified, according to structures thereof, into a tandem type in which a plurality of image forming portions are arranged in tandem and a rotary type in which a plurality of image forming portions are arranged cylindrically. Transfer systems include a direct transfer system in which a toner image is transferred directly from a photosensitive member to a sheet and an intermediate transfer system in which a toner image is once transferred to an intermediate transfer member and then to a sheet.
Among such image forming apparatuses, apparatuses targeted to the printing market of small circulation publications have been provided in recent years by taking the advantage that plates are not made in the apparatuses. However, the apparatuses have to achieve high image quality in order to be accepted in the quick printing market, and importance is put on the accuracy of an image position with respect to a sheet as one of the factors contributing to the image quality. The accuracy of an image position includes misalignment between a front surface image and a rear surface image when a two-sided image is formed. According to an analysis of the factors for the purpose of improving the accuracy of an image position with respect to a sheet, the accuracy is determined by registration in the sheet conveying direction, registration in the width direction perpendicular to the sheet conveying direction, magnification and skew feeding. Only skew feeding among these factors is difficult to correct by electrical control. For example, it is possible to correct an image position with respect to a sheet by detecting skew feeding of the sheet and forming an image which is inclined correspondingly to the skew. However, particularly in the case of a color image formed by superimposing three or four colors, if the image is inclined for each sheet, the tint will vary for each sheet due to misalignment in dot formation of the respective colors. In addition, the time is required for calculation to incline the image, which results in significant decrease in productivity. Therefore, it is desirable to deal with the skew feeding by improving the sheet conveyance accuracy (skew correction accuracy).
In the related art, there is a method of correcting skew feeding by causing a side edge of a sheet to abut a side reference guide, which is arranged along the sheet conveying direction, by an obliquely conveying roller as one of methods for correcting skew feeding of sheets. According to this method, right and left side edges of a sheet are not reversed. Accordingly, this method is advantageous in that the skew feeding can be corrected using the same reference for both the first and second faces of the sheet.
In addition, the market requires that a wide variety of materials be supported. For example, various materials such as sheets having a basis weight of 40 [g/m2] or more and 350 [g/m2] or less, coated sheets and film sheets are required. Here, this method is described with reference to FIG. 9. FIG. 9 is a side view illustrating a conventional obliquely feeding conveyance unit as viewed in the sheet conveying direction. When a sheet S is caused to abut an abutting reference surface 311 of an abutting reference guide portion 31, a guiding shape is also necessary to be formed in the vertical direction so that the sheet S does not escape in the vertical direction. Accordingly, the abutting reference guide portion 31 is formed to have a U-shaped cross section as illustrated in FIG. 9. Normally, a gap between an upper guide 312 and a lower guide 313 illustrated in FIG. 9 is often about twice as wide as the thickness of the thickest sheet. A sheet S is conveyed in a manner that the sheet S is pressed to the abutting reference guide portion 31 by a skew correction roller 32 and a driven roller 34 opposed to the skew correction roller 32 to correct the skew feeding of the sheet S. If the gap between the upper guide 312 and the lower guide 313 of the abutting reference guide portion 31 is too small, the conveyance resistance when a sheet S is conveyed becomes higher. If the gap is too large, on the other hand, a sheet S is buckled or forms a loop in the gap, which causes misalignment of the sheet by an amount corresponding to the buckling or the loop.
Therefore, there is proposed, for example in Japanese Patent Laid-Open No. 2002-356250, an apparatus adapted to detect the thickness of a sheet and control the gap between upper and lower guides of a reference guide portion according to the sheet thickness in order to prevent buckling or formation of a loop when sheets having different thicknesses are conveyed.
In the technique of changing the width of the gap between the upper and lower guides according to the sheet thickness, one of the upper guide and the lower guide is formed of another member from the reference guide and the upper or lower guide position is adjustable. In such a structure, the abutting reference surface 311 and the lower guide 313 are formed integrally and the upper guide 312 is formed of another member from the abutting reference surface 311 as illustrated in FIG. 10. In such a structure, however, a joint 315 of the abutting reference surface 311 and the upper guide 312 is formed vertically on an upper portion of the abutting reference surface 311 as illustrated in FIG. 10. Accordingly, if a side edge of a sheet S abuts the joint 315, the sheet S may be caught in the joint and the side edge may be damaged or cut. The damage or cut is more likely to occur particularly in a thin sheet having low stiffness (rigidity). This is because if the sheet has low stiffness, the sheet is buckled to form a loop when the sheet is caused to abut the abutting reference guide portion 31 by the skew correction roller 32, and thus the side edge of the sheet is likely to be oriented upward or downward along the extending direction of the joint 315.
In the example structure illustrated in FIG. 10, a nip line N passing through a nip portion between the skew correction roller 32 and the driven roller 34 is coincident with a center line C of the gap between the upper guide 312 and the lower guide 313 as viewed in the conveying direction, similarly to the example structure illustrated in FIG. 9. Accordingly, the sheet S is conveyed in a manner that the leading edge (side edge) thereof follows the surface of the lower guide 313 under the influence of gravity. When the sheet S abuts the abutting reference surface 311 in this state, the side edge escapes upward as illustrated in FIG. 10. At this time, since the joint 315 of the abutting reference surface 311 and the upper guide 312 extends vertically above the side edge, the side edge of the sheet is likely to be caught in the joint 315.
Normally, parameters for preventing buckling or formation of a loop depend on the stiffness of the sheet. FIG. 11 is a graph in which the relation between the sheet thickness (paper thickness) of various materials and the stiffness is plotted. It can be seen in the graph that the stiffness varies considerably among materials having the same thickness. The relation therebetween shows an exponential trend in which a thin sheet tends to have extremely low stiffness. Therefore, it is difficult to prevent buckling or formation of a loop by simply narrowing the gap between the guides for such a sheet. As described above, a side edge of a thin sheet, in particular, is likely to be caught in the joint 315 when a loop is formed, and appropriate skew correction cannot be made. Accordingly, the accuracy of the image position is lowered. In addition, when an excessive loop is formed, the sheet is buckled, which makes a jam more likely to occur.
The present invention is directed to a sheet conveying apparatus in which a side edge of a sheet, in particular a thin sheet having a low stiffness, is prevented from being damaged when the sheet is caught in a joint between a guide member and an abutting reference surface at a reference guide portion, and to an image forming apparatus which includes the sheet conveying apparatus.